/Engine/Source/Runtime/Engine/Classes/Engine/Engine.h
Notification of server travel error messages, generally network connection related (package verification, client server handshaking, etc) allows the engine to handle the failure
virtual void HandleTravelFailure ( [UWorld](API\Runtime\Engine\Engine\UWorld) * InWorld, [ETravelFailure::Type](API\Runtime\Engine\Engine\ETravelFailure__Type) FailureType, const [FString](API\Runtime\Core\Containers\FString) & ErrorString )